Locking brakes

[mikes67]

I have a 69 C20 4wd that had the front drum system replaced with a 71 disk brake system. Under normal conditions the pickup stops perfectly, but in a panic situation the back brakes lock up before the front. I've replaced the master cylinder, power booster, and the porportioning valve. Not sure what size the rear brakes are, but they are standard for the pickup. The lugs on the front are bigger than the rear.

Any idea on what I can look at?

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